Many circuits are capable of converting or transforming an electric signal where the regulation of an output signal is ensured by switching one or several switches at a relatively high frequency, for example, greater than 1 kHz. The switches, for example, are transistors (MOS, IGBT, etc.) used in switched mode. By varying the switching frequency and duty cycle, it is possible to control the characteristics of the output signal (shape, frequency, power, voltage, etc.). Circuits of this type comprise, among others, switched-mode power supplies, D.C.-D.C. converters, circuits for correcting the power factor, inverters, converters for the generation of solar or wind electric power, etc.
FIG. 1 shows an example of a three-phase inverter circuit 10. In operation, circuit 10 receives a D.C. input voltage between two input terminals A and B, and delivers a three-phase A.C. signal on three output terminals C, D, and E. In this example, circuit 10 is used to power a three-phase asynchronous motor M.
Circuit 10 comprises six switches K1 to K6, for example, insulated-gate bipolar transistors (IGBT). Switches K1 and K2 are series-connected between terminals A and B. Switches K3 and K4 on the one hand, and K5 and K6 on the other hand, are also series-connected between terminals A and B in parallel with switches K1 and K2. Output terminals C, D, and E are respectively connected to the common node between switches K1 and K2, to the common node between switches K3 and K4, and to the common node between switches K5 and K6. Diodes D1 to D6 are respectively forward-connected between terminal C and terminal A, between terminal B and terminal C, between terminal D and terminal A, between terminal B and terminal D, between terminal E and terminal A, and between terminal B and terminal E. The control gates of switches K1 to K6 are respectively connected to output terminals o1 to o6 of a control circuit 12 (MCU), for example, a microcontroller.
In operation, microcontroller 12 imposes to switches K1 to K6 a switching sequence capable of transforming the D.C. voltage, applied on terminals A and B, into an averaged three-phase A.C. voltage provided on terminals C, D, and E. The switching frequency of the switches and the duty cycle of the switching may be dynamically modified (for example, via a user interface) to modify the characteristics of a three-phase A.C. signal supplied to the motor and thus vary the rotating speed thereof (speed variator). It should be noted that such a circuit may also be used to convert the electric power generated by a current generator.
Diodes Di (with i ranging from 1 to 6) are so-called free-wheel diodes, enabling to ensure the continuity of the current in the inductive elements of motor M on turning-off (blocking) of switches Ki. Diodes Di especially enable to avoid for abrupt voltage peaks to be applied across the switches on each turning-off of a switch Ki.
Generally, in conversion circuits using a chopper switch, a diode is often associated with the switch to provide a secondary conduction path to the current when the switch is turned off.
A disadvantage of converters using a chopper switch in hard switching associated with a free wheel diode (PN junction) (for example, in an assembly comprising an inductive element, a chopper switch, and a free wheel diode) is the power loss due to a charge recovery phenomenon occurring each time the diode switches from an on state to an off state (that is, on each turning-on of the switch).
FIG. 2 schematically shows the time variation of currents ITR and ID, respectively in a chopper switch (transistor) and in a diode associated with this switch, on blocking of the diode (turning-on of the switch). It is assumed that at a time t0, the switch is controlled to be turned on. The switch starts conducting. Current ID in the diode then decreases from a value IF corresponding to the forward current through the diode before blocking, to a negative value IRM (reverse mode current), before increasing to cancel. The variation of current ITR in the switch is opposite to that of current ID, that is, current ITR increases from an approximately zero value (off switch) to a maximum value (peak) before decreasing to a stabilized value IF (on switch). Duration trr between a time t1, subsequent to t0 and a time t2, subsequent to t1, during which the diode conducts a reverse current (and the switch conducts a current greater than the steady-state current), is called reverse recovery time. This duration is necessary to drain off the remaining stored charges when the forward current cancels (time t1). Such remaining charges are called recovered charges. Quantity Qrr of recovered charges can be defined as being the time integration of the reverse current crossing the diode on blocking thereof (hatched area in FIG. 2). Quantity Qrr of recovered charges depends on several factors and, in particular, on the intensity of current IF crossing the diode on blocking thereof, on the current decrease slope on blocking thereof (linked to the current growth slope in the transistor when it is turned on, that is, in time interval toff between time t0 and time t1), on the junction temperature in operation, and on the voltage applied in reverse mode to the diode to block it. This recovery phenomenon characteristic of PN junctions is all the greater as the reverse breakdown voltage of the diode is high. The recovered charges are dissipated in heat in the switch and do not take part in the output signal generation. The loss due to recovered charges amounts for a significant part of the total loss in converters of this type.
It would be desirable to decrease the power loss in converters using one or several chopper switches and, more specifically, the loss due to the charge recovery phenomenon in diodes associated with chopper switches. More generally, it would be desirable to optimize the operation of converters using at least one chopper switch associated with a free wheel diode.